ACID means Atomic, Consistent, Isolated, and Durable. It's a term used when talking about databases, and the guarantees they give to users about transactions.
Atomic means that a whole transaction either succeeds or fails entirely, so that the database is not left in some half-way-through state.
Consistent means that all data rules and constraints set up in the database are enforced before and after each transaction.
Isolation means that transactions that run in parallel don't know about each other, and can't reach each other's changes before they're committed.
Durable means that once the data is committed, you can rely on it staying that way, even if the database system or the whole OS crashes.